1. Field of the Invention
The present invention relates to a method of monitoring the accuracy with which a photomask is transferred or the accuracy with which a pattern is directly written onto a wafer, the photomask and the wafer being used for manufacturing semiconductor devices.
2. Description of the Related Art
When semiconductor devices such as ICs are manufactured, a photo-lithography technique is used to form patterns. As the degree of integration of semiconductor devices has increased in recent years, manufacturing processes for semiconductor devices have become complex, and there has been a demand that accuracy of photomasks, such as pattern accuracy and overlay accuracy, be improved. For this reason, an electron beam exposure system has been employed which easily processes patterns and is capable of rapidly and highly accurately forming patterns on a photomask. Various types of electron beam exposure systems have been employed. Instead of an exposure system using a conventional spot beam raster scan method, an exposure system using a variable shaped beam vector scan method has been employed, which method can form patterns more accurately than the conventional method.
An exposure method based on the variable shaped beam vector scan method will now be described. As shown in FIG. 9, a photomask 21 has a plurality of chip portions 23. Each chip portion 23 is partitioned into small zones or fields 30 of a predetermined size, as depicted in FIG. 10. The small zones 30 in the chip portions 23 are exposed one after another, and thus all of the chip portions 23 are exposed, whereby the entire surface of the photomask 21 is consequently patterned.
As shown in FIG. 11, each field 30 is partitioned further into small zones or shots 31. The chip portions 23 are exposed shot-by-shot. Each field 30 is usually partitioned into rectangular shots 32, however, a trapezoidal FIG. 33 may also be formed due to an oblique pattern or other factors. In such a case, to precisely form the oblique portion of the trapezoidal FIG. 33, as shown in FIG. 12, the trapezoidal FIG. 33 is partitioned further into small rectangular shots 34, which are exposed one by one so that a part of one rectangular shot 34 is overlapped by a part of another rectangular shot 34.
Thus, pattern data in one chip portion 23 is divided into a large number of fields 30, and pattern data in one field 30 is further divided into a large number of shots 31. These fields 30 and shots 31 are then exposed. For this reason, the stitching accuracy between adjacent fields 30 within each chip portion 23 and the dimensional accuracy of patterns within the fields 30 in which the shots 31 are combined, become crucial to the operation of electron beam exposure systems. Thus it is required to maintain these accuracies at a level which is as high as possible.
However, because there has hitherto been no method of measuring and inspecting the accuracy with which patterns are written (hereinafter referred to as a writing accuracy), it is difficult to control pattern formation with high accuracy, thus making it difficult to improve the yield of products.